This invention relates generally to signal generators and in particular to a digital baseband modulator for generating digitally modulated signals.
Wireless communications, including digital cellular telephones and personal communications service (PCS) telephones, are rapidly becoming a major sector of the communications industry. Emerging wireless communications technologies are a driving force behind the rapidly expanding number of digitally modulated signals that must now be accommodated by signal generators, both in the design and manufacturing environments. The wireless system designer who must choose a particular type of digital modulation is faced with a number of challenges. The wireless system must allow for signal strengths that vary over time and location, with multipath, fading and interference.
Wireless handsets are increasingly smaller in size and have limited battery capacity. At the same time, user demands continue to increase for higher data rates, better voice quality, fewer dropped calls, and longer talk times. Designing, manufacturing, and maintaining wireless devices, including both handsets and base stations, requires appropriate test equipment such as signal generators which are capable of generating test signals for precisely simulating real world conditions as well as known signals that conform to industry standards.
Wireline systems, which include cable television and telephone applications, have similarly expanded the types of digital communications to many of the same problems encountered in wireless applications. A discussion of various digitally modulated signal types that exist may be found in "Digital Communication, Second Edition", Lee, Edward, A. and Messerschmitt, David G., Kluwer Academic Publishers, Mass., 1994.
Signal generators must be versatile and adaptible enough to handle both existing and newly created modulation types. During the development of a new wireless system, designers may be faced with the problem of not having a receiver to verify the operation of the newly designed signal generator and no signal generator capable of verifying the operation of the newly designed receiver. A versatile and adaptible signal generator will help in avoiding this dilemma by providing a known test signal by which the performance of the new receiver can be determined without developing specialized signal sources.
Most wireless communications systems now transmit information as digital data. Analog information such as digitized voice is transmitted in the same manner as other digital data. Because the digital information consists of binary 1s and 0s, the digitally modulated radio frequency (RF) carrier has a finite set of modulation states. These modulation states may be defined according to variations in amplitude, phase, or frequency of the RF carrier. In addition, other types of modulation utilizing both amplitude and phase information in the in-phase (I) and quadrature (Q) signals of the RF carrier have been employed to obtain quadrature amplitude modulation (QAM). QAM is one form of IQ modulation.
RF signal generators having built-in modulation capabilities for testing communications systems have existed for many years. Such signal generators include synthesized RF sources that generate RF carrier signals at desired frequencies, such as in the 824-894 MegaHertz (MHz) frequency range for North American cellular telephone. An IQ modulator provides for modulating the RF carrier signal in response to separate I and Q signals which are generated by a baseband modulator.
Prior art baseband modulators were implemented on an ad hoc basis to accommodate new digital modulation signal types. Adding new types of digital modulation signal required additional components and new hardware configurations within the RF signal generator, greatly increasing its cost and complexity. Baseband modulators having customized digital filters and symbol builders were assembled to comply with widely varying digital modulation signal types. Such specialized hardware significantly increased the cost of supporting additional types of digital modulation signal types in the RF signal generator.
As the number of digital modulation signal types continues to rapidly expand, the ability of the baseband modulator to adapt and accommodate these new signal types has become inadequate. Therefore, it would be desirable to provide a digital baseband modulator for RF signal generators that is readily adaptible to the wide variety of existing as well as emerging digital modulation signal types. It would be further desirable that the digital baseband modulator be implemented with lower cost, require less space, and be adaptible with changes in software rather than in hardware to accommodate different types of digital modulation.